The present invention relates in general to a thin film electroluminescent display device and is concerned, more particularly, with an improved dark field material for such a thin film electroluminescent display device.
Electroluminescent devices generally comprise a phosphor layer disposed between two electrode layers with one of the electrodes being transparent so as to permit viewability of the phosphor layer. It is known to provide a dark field layer behind the phosphor layer in order to improve the contrast ratio of the device when using a segmented back electrode layer; that is to say, to provide visibility of the phosphor layer overlying the back electrode segments even under ambient conditions of high brightness. See U.S. Pat. No. 3,560,784 for an example of a dark field layer, the material of which may comprise arsenic sulphide, arsenic selenide, arsenic sulfoselenide or mixtures thereof. However, these arsenic compounds either do not provide a satisfactory dark color or they change color during use.
Perhaps the most common dark field material presently being used is cadmium telluride (CdTe). Although the CdTe layer provides for enhancement in contrast between the displayed information and the background, one of the problems associated with the CdTe composition is that it is toxic and the material does not meet safety specifications for commercial products as required by OSHA (Occupational Safety and Health Act).
One solution to this toxicity problem is described in copending application U.S. Ser. No. 262,097, filed May 11, 1981 and assigned to the present assignee, which defines an electroluminescent device having a dark field layer comprising a cermet of chromium oxide -chromium (Cr.sub.2 O.sub.3 /Cr). Although overcoming the toxicity problem, this cermet comprises a combination of a metal (Cr) and an oxide (Cr.sub.2 O.sub.3) of the same base metal, thereby rendering the dark field composition difficult, if not impossible, for analysis of the constituent proportions. Such analysis is important to enable precise control of the constituent proportion for providing optimum results.
Accordingly, it is an object of the present invention to provide an improved electroluminescent display device and in particular an improved dark field material for such a device.
A further object of the present invention is to provide an improved dark field in accordance with the preceding object and which is characterized by an enhanced brightness of the phosphor carried out by temperature control which has been found to be a function of the composition of the dark field layer.
Another object of the present invention is to provide an improved dark field in accordance with the preceding objects and which is characterized by an improved contrast ratio of the device.
Still another object of the present invention is to provide a dark field material in accordance with the preceding objects and which is non-toxic and meets the safety specifications for commercial products required by OSHA.
A further object of the present invention is to provide an improved dark field layer in a thin film electroluminescent display device in which for at least some applications, only a single transparent dielectric layer of the device is employed in comparison with the typical first and second transparent dielectric layers used in the past in electroluminescent thin film display devices.
Still a further object of the present invention is to provide an improved dark field material for a thin film electroluminescent display device in which the dark field layer is formed of constituents which are readily analyzable, and thus precisely controllable, to provide enhanced flexibility in controlling parameters of the dark field layer such as contrast ratio.